This invention relates generally to multiple CRT projection type television systems, and particularly relates to a system for preventing CRT phosphor burn in a multiple CRT projection type television system.
A common type of projection television system includes three cathode ray tubes disposed side by side relative to one another each emitting a light beam of one of the three different primary colors, red, green and blue, onto a generally planar projection screen through a lens assembly. The three different color light beams form a proper colored picture when they are projected onto the projection screen in such a manner that images of different color are superimposed upon one another without any displacement.
The three cathode ray tube (CRT) projection color television receiver approach as well as other projection television system approaches have suffered from inherent limitations in the brightness of the reproduced image on the projection screen. These limitations have necessitated a darkened viewing environment for optimum projection television receiver viewing. Other equally undesirable conditions from the viewer's standpoint have been caused by limitations in video display brightness and contrast in a projection television receiver. These limitations result from various characteristics of the projection television environment such as light losses in the image projection lens system, absorption and dispersion of the light energy containing image information incident upon the projection screen, reflection losses if a mirror system is used, and the large size of the projection screen relative to the total surface area of the three CRT's employed in the projection television system. To compensate for these inherent projection television system limitations, substantial effort has been expended to enhance the images produced by the individual CRT's.
The principle elements in the tubes are an electron gun, a target coated with a phosphor upon a surface directly facing the impinging electron beam, various mirror and/or lens combinations to achieve the desired optical effects, and a transparent faceplate through which the image is projected upon a screen external to the CRT's envelope. The most common approach to increasing image intensity, or brightness, is to increase the inter-electrode voltages which accelerate the electrons toward the phosphor-coated faceplate. This has resulted in an increased danger of permanently damaging the CRT's screen by means of a continuous beam of undeflected, energetic electrons impinging on the same spot, or small area, when deflection voltages are removed. During normal operation the electron beam is swept vertically and horizontally across the CRT faceplate. Should a vertical or horizontal deflection yoke malfunction or be deprived of appropriate deflection signals during operation, a permanent mark will be "burned" on the phosphor coating of the CRT's faceplate in the form of a spot or a horizontal or vertical line. This is not a new problem brought on by the advent of projection television systems, for there is much in the prior art addressing this problem in conventional television receivers, but it is only of recent that this problem has been dealt with in the context of a projection television system environment.
One approach to protecting the projection television system's CRT faceplate is disclosed in Quasar Electronics Company Service Manual for projection television chassis AMVDTS-800 published in 1978 by Matsushita Electric Corporation of America. FIG. 1 shows the horizontal deflection yoke open circiut detector system described in a detailed schematic in the referenced Quasar Service Manual. Under normal conditions with current flowing through green 13, blue 15 and red 17 deflection yokes, the magnetic effects in transformer 11 and 19 caused by the current flowing through coils L2 and L3 in each of the transformers cancels out. Thus, under normal operating conditions no current is induced in windings L1 of transformers 11 and 19. With no composite pulse induced in the L1 windings, diode 21 which is connected to them in series detects no voltage. However, during abnormal operation of any of the three deflection yokes, an output voltage is provided to diode 21 in the following manner. If blue deflection yoke 15 ceases to induce a positive pulse in the L3 winding of transformer 11, the L1 winding in that transformer becomes unbalanced because of the presence of only the positive pulse induced by L2. Since transformer 19 does not include a winding from blue deflection yoke 15, L1 in transformer 19 remains balanced. Diode 21 detects and rectifies only the positive pulse form from the L1 winding to transformer 11 and this output is used to control beam current shutoff circuitry. Failure of either the red or green deflection yokes 17 or 13, respectively, will produce the same unbalanced flux condition in transformers 11 and 19 producing a resultant output pulse used to drive CRT shutdown circuitry. Transformer 23 is used to detect failure of all three deflection yokes and to generate an output signal for CRT shutdown in response thereto. This approach thus requires three separate transformers which reduces its reliability.
These and other problems encountered in the prior art are avoided by the present invention which provides a system for preventing CRT phosphor burn in a multiple CRT projection type television system by means of a single transformer in each of the vertical and horizontal deflection systems.